Navi Pillay, the United Nations human rights chief, has blamed both sides in the Syrian conflict of being guilty of what may amount to war crimes and crimes against humanity, and called for the international community to take "protective action".
Speaking to the UN human rights council in Geneva on Monday, Pillay was harsh in her criticism of both sides, but stopped short of calling for a no-fly zone or military intervention.
"The use of heavy weapons by the government and the shelling of populated areas have resulted in high numbers of civilian casualties, mass displacement of civilians inside and outside the country and a devastating humanitarian crisis," Pillay said.
She was equally concerned at violations by anti-government forces, including murder, extra-judicial execution and torture, and an increased use of improvised explosive devices.
Pillay, a former UN war crimes judge, has repeatedly called for alleged crimes in Syria to be referred to the International Criminal Court.
Such a referral can only be made through the UN Security Council, which is split on what action the international community should take, if any, on what has become an increasingly violent uprising against the government of Syrian President Bashar al-Assad.
"A referral will make it abundantly clear to all actors in Syria that they will not escape justice and will be held accountable for alleged violations of international human rights law and international humanitarian law," Pillay said on Monday in a separate speech.
"Opposition forces should be under no illusion that they will be immune from prosecution," she added.
Amateur video posted on YouTube on Monday showed images of 20 dead Syrian soldiers, blindfolded and handcuffed, after they were apparently executed in the northern city of Aleppo.
The Syrian Observatory for Human Rights, an anti-government rights group, said that the killings had occurred in the Hanano district of the city at some point over the weekend.
It was not possible to verify the video's authenticity.
The Human Rights Council has repeatedly condemned Syria's government for its handling of an uprising that began as a peaceful protest movement but has now escalated into a civil war.
China, Russia and Cuba have consistently voted against its resolutions.
Independent UN investigators, in a report last month, said that Syrian government forces and allied militia have committed war crimes including murder and torture of civilians in what appears to be state-directed policy.
Meanwhile, violence continued in Syria on Monday, with reports of the executions accompanied by air raids on Aleppo. Government forces used MiG warplanes and helicopters to assault several areas of the city, the AFP news agency's correspondent reported.
In-depth coverage of escalating violence across Syria
The SOHR, a UK-based group, said that at least five people had died in morning bombing raids on the Marjeh, Sakhur, Hanano, Tariq al-Bab and Sheikh Khodr neighbourhoods of Aleppo, among a total of 95 civilians killed nationwide. Sixty-three of the dead were civilians, the SOHR said.
On the diplomatic front, Lakhdar Brahimi, the joint UN-Arab League envoy on the conflict, was in Cairo on Monday for talks with Egyptian and Arab League officials, ahead of a planned trip to Damascus.
Russia, an ally of the Syrian government, called on Monday for a comprehensive peace agreement to be drawn up at a conference involving all domestic participants in the conflict.
"We are proposing to our Western partners the organisation of a 'Taif conference' between all the players of the conflict," Mikhail Bogdanov, a Russian deputy foreign minister, said, alluding to a 1989 peace deal signed in the Saudi city between the parties to the Lebanese civil war.
"This conference should bring together opposition and regime figures, as well as Christian, Alawite and Druze community members," Bogdanov added.
Meanwhile, in neighbouring Lebanon, security forces released four Syrian hostages in a raid in Beirut.
Mahed Meqdad, a spokesman for the clan that was holding the hostages, told Al Jazeera that the Lebanese army had raided four locations in southern Beirut at dawn on Tuesday.
The four Syrians were taken by Lebanese forces after a gun battle in which, Meqdad said, a Turkish captive was wounded.
The Meqdad family kidnapped several Syrians and a Turkish national after a member of their family was kidnapped by the armed Syrian opposition in Damascus.